| Certain FIDL types can be made *optional* with no change to the wire shape of |
| their containing message with the addition of the `:optional` constraint. |
| Further, the `table` layout is always optional, while the `struct` layout never |
| is. To make a `struct` optional, it must be wrapped in a `box<T>`, thereby |
| changing the wire shape of its containing message. |
| |
| | Base type | Optional version | Does optionality change the wire layout? | |
| |---|---|---| |
| | `struct {...}` | `box<struct {...}>` | Yes | |
| | `table {...}` | `table {...}` | No | |
| | `union {...}` | `union {...}:optional` | No | |
| | `vector<T>` | `vector<T>:optional` | No | |
| | `string` | `string:optional` | No | |
| | `zx.handle` | `zx.handle:optional` | No | |
| | `client_end:P` | `client_end:<P, optional>` | No | |
| | `server_end:P` | `server_end:<P, optional>` | No | |
| |
| All other types (`bits`, `enum`, `array<T, N>`, and the primitive types) cannot |
| be made optional. |
| |
| <<../examples/key_value_store/_callout.md>> |
| <<../examples/key_value_store/_support_trees_tutorial.md>> |
